A study of pollutant dispersion near highways

Abstract

As a part of a major roadway dispersion project undertaken by the New York State Department of Environmental Conservation, several tracer release experiments were conducted on an open, at-grade highway site. The observed concentration profiles indicate that the tracer concentration decreases with increasing distance from the roadway more rapidly for parallel wind-road orientation than for perpendicular wind-road orientation. However, the decrease of concentration with height on the tower at the nearest, downwind roadside receptor is slow for parallel case as compared to perpendicular case. Four Gaussian dispersion models (HIWAY, GM, CALINE-2, AIRPOL-4) and four numerical models (DANARD, MROAD 2, RAGLAND, ROADS) are used to predict the tracer gas concentrations. Of the models tested, GM and HIWAY perform well compared to other Gaussian models. The numerical models performed about the same as the above two Gaussian models. The GM model provides a better simulation by far for parallel wind cases than any of the other models tested. The computed wind flow patterns indicate upward motions over the roadway in parallel wind cases which may be due to the mechanical turbulence generated by the traffic flow. The dispersion parameter values computed from the concentration measurements agree very closely to those prescribed for neutral conditions in the GM model and unstable conditions in the HIWAY model. It is suggested that the stability adjacent to the roadway may be determined either through bulk Richardson number, or wind fluctuation data obtained at the site. Further, a better formulation of the dispersion parameters may be σ z ~ σ σ x, and σ y ~ σ θ x, rather than the power law relationship ( σ z ~ α 1 x b 1 and σ y ~ α 2 x b 2 ) commonly used in the existing highway dispersion models. In this regard, wind fluctuation statistics sampled for a duration of one hour are appropriate for describing the dispersion mechanism near roadways.